Various techniques have been developed for multiplexing analog television (TV) signals. Time Compression Multiplexing (TCM) is one such method where various signals are multiplexed by means of time compressing their analog waveforms into segments in such a way that the compressed segments from different sources can be sent on the same channel in separate time intervals known as time division multiplexing. Articles such as, for example, "TV Bandwidth Compression Techniques Using Time Companded Differentials and Their Application To Satellite Transmissions" by K. Y. Eng et al in BSTJ, Vol. 61, No. 10, December 1982 at pages 2917-2927 have discussed various properties and ways to implement TCM in the transmission of multiple TV signals through a single satellite transponder.
More recently, developments have been made available to permit three broadcast-quality TVs to be transmitted through a satellite transponder using either the TCM or a Time-Frequency Multiplexing (TFM) technique. With such TCM and TFM systems, one requirement is that the input TV signals be synchronized, at least to the extent that their vertical blanking intervals overlap. If the signals to be synchronized are colocated in the same uplink earth station, then frame synchronizers could be used. However, if such TV signals are to be transmitted from separate earth stations, then the uplinks have to be synchronized to ensure that the signal bursts from different sources would arrive at the satellite without overlap.
Synchronization techniques in communication satellite systems have been developed in the past years dealing primarily with digital TDMA applications. One such technique is disclosed in U.S. Pat. No. 3,646,444 issued to W. Bitzer on Feb. 29, 1972. There, a reference ground station transmits a fixed frequency reference signal which is received by every other ground station. Each of these other ground stations then uses the received reference signal to generate a separate local signal which is separately transmitted to the satellite and received back again by the originating ground station. The received local signal is then used to synchronize this other ground station. For other similar techniques, see for example, U.S. Pat. No. 3,562,432 issued to O. G. Gabbard on Feb. 9, 1971; U.S. Pat. No. 3,761,814 issued to G. Bernasconi et al on Sept. 25, 1973; U.S. Pat. No. 3,878,339 issued to W. G. Maillet on Apr. 15, 1975; U.S. Pat. No. 4,320,503 issued to A. Acampora on Mar. 16, 1982 and the article "Synchronization Methods for TDMA" by P. P. Nuspl et al in Proceeding IEEE, Vol. 65, No. 3, March 1977, at pages 434-443.
However, these previous synchronization techniques were designed for performance far exceeding the present requirement for synchronizing analog TV signals and hence tend to be more complicated than needed. More importantly, these techniques were meant for digital signals and are not suitable for analog TV where the color subcarrier and various sync pulses must bear strict phase and frequency relationships and thus cannot be advanced or retarded with respect to one another arbitrarily. Therefore, the problem, remaining in the prior art is to provide a simple synchronization technique for synchronizing analog TV signal transmissions at a satellite.